Solenoids are used in many applications, such as locking systems on electric brake actuators. They are often devices with two discrete positions, that stay in each state even with power removed, referred to as bistable devices. A solenoid may change state in response to electrical energy applied to the device. Some aircraft brakes use solenoids to lock brake actuators in place, for example. Such a solenoid may act as a parking brake. Solenoids may also be used in other applications on aircraft.
In many applications it may be beneficial to know the state of a solenoid. However, in many applications the state of the solenoid may not be available. Adding additional hardware to monitor solenoid state may add to the weight of an aircraft, which in turn may impede aircraft performance.